Drug-Metabolizing Proteins
Induction of drug-metabolizing enzymes (“DMEs”) is a common biological response to xenobiotics, the mechanisms and consequences of which are important in academic, industrial, and regulatory areas of pharmacology and toxicology.
For most drugs, drug-metabolizing enzymes determine how long and how much of a drug remains in the body. Thus, developers of drugs recognize the importance of characterizing a drug candidate's interaction with these enzymes. For example, polymorphisms of the drug-metabolizing enzyme CYP2D6, a member of the cytochrome p450 (“CYP”) superfamily, yield phenotypes of slow or ultra-rapid metabolizers of a wide spectrum of drugs including antidepressants, antipsychotics, beta-blockers, and antiarrhythmics. Such abnormal rates of drug metabolism can lead to drug ineffectiveness or to systemic accumulation and toxicity.
For pharmaceutical scientists developing a candidate drug, it is important know as early as possible in the design phase which enzymes metabolize the drug candidate and the speed with which they do it. Historically, the enzymes on a drug's metabolic pathway were determined through metabolism studies in animals, but this approach has now been largely supplanted by the use of human tissues or cloned drug-metabolizing enzymes to provide insights into the specific role of individual forms of these enzymes. Using these tools, the qualitative and quantitative fate of a drug candidate can be predicted prior to its first administration to humans. As a consequence, the selection and optimization of desirable characteristics of metabolism are possible early in the development process, thus avoiding unanticipated toxicity problems and associated costs subsequent to the drug's clinical investigation. Moreover, the effect of one drug on another's disposition can be inferred.
Known drug-metabolizing enzymes include the cytochrome p450 (“CYP”) superfamily, N-acetyl transferases (“NAT”), UDP-glucuronosyl transferases (“UGT”), methyl transferases, alcohol dehydrogenase (“ADH”), aldehyde dehydrogenase (“ALDH”), dihydropyrimidine dehydrogenase (“DPD”), NADPH:quinone oxidoreductase (“NQO” or “DT diaphorase”), catechol 0-methyltransferase (“COMT”), glutathione S-transferase (“GST”), histamine methyltransferase (“HMT”), sulfotransferases (“ST”), thiopurine methyltransferase (“TPMT”), and epoxide hydroxylase. Drug-metabolizing enzymes are generally classified into two phases according to their metabolic function. Phase I enzymes catalyze modification of functional groups, and phase II enzymes catalyze conjugation with endogenous substituents. These classifications should not be construed as exclusive nor exhaustive, as other mechanisms of drug metabolism have been discovered. For example, the use of active transport mechanisms been characterized as part of the process of detoxification.
Phase I reactions include catabolic processes such as deamination of aminases, hydrolysis of esters and amides, conjugation reactions with, for example, glycine or sulfate, oxidation by the cytochrome p450 oxidation/reduction enzyme system and degradation in the fatty acid pathway. Hydrolysis reactions occur mainly in the liver and plasma by a variety of non-specific hydrolases and esterases. Both deaminases and amidases, also localized in the liver and serum, carry out a large part of the catabolic process. Reduction reactions occur mainly intracellularly in the endoplasmic reticulum.
Phase II enzymes detoxify toxic substances by catalyzing their conjugation with water-soluble substances, thus increasing toxins' solubility in water and increasing their rate of excretion. Additionally, conjugation reduces the toxins' biological reactivity. Examples of phase II enzymes include glutathione S-transferases and UDP-glucuronosyl transferases, which catalyze conjugation to glutathione and glucuronic acid, respectively. Transferases perform conjugation reactions mainly in the kidneys and liver.
The liver is the primary site of elimination of most drugs, including psychoactive drugs, and contains a plurality of both phase I and phase II enzymes that oxidize or conjugate drugs, respectively.
Physicians currently prescribe drugs and their dosages based on a population average and fail to take genetic variability into account. The variability between individuals in drug metabolism is usually due to both genetic and environmental factors, in particular, how the drug-metabolizing enzymes are controlled. With certain enzymes, the genetic component predominates and variability is associated with variants of the normal, wild-type enzyme.
Most drug-metabolizing enzymes exhibit clinically relevant genetic polymorphisms. Essentially all of the major human enzymes responsible for modification of functional groups or conjugation with endogenous subsituents exhibit common polymorphisms at the genomic level. For example, polymorphisms expressing a non-functioning variant enzyme results in a sub-group of patients in the population who are more prone to the concentration-dependent effects of a drug. This sub-group of patients may show toxic side effects to a dose of drug that is otherwise without side effects in the general population. Recent development in genotyping allows identification of affected individuals. As a result, their atypical metabolism and likely response to a drug metabolized by the affected enzyme can be understood and predicted, thus permitting the physician to adjust the dose of drug they receive to achieve improved therapy.
A similar approach is also becoming important in identifying risk factors associated with the development of various cancers. This is because the enzymes involved in drug metabolism are also responsible for the activation and detoxification of chemical carcinogens. Specifically, the development of neoplasia is regulated by a balance between phase I enzymes, which activate carcinogens, and phase II enzymes, which detoxify them. Accordingly, an individual's susceptibility to cancer often involves the balance between these two processes, which is, in part, genetically determined and can be screened by suitable genotyping tests. Higher induction of phase I enzymes compared to phase II enzymes results in the generation of large amounts of electrophiles and reactive oxygen species and may cause DNA and membrane damage and other adverse effects leading to neoplasia. Conversely, higher levels of phase II enzyme expression can protect cells from various chemical compounds.
Abnormal activity of drug-metabolizing enzymes has been implicated in a range of human diseases, including cancer, Parkinson's disease, myetonic dystrophy, and developmental defects.
Cytochrome p450
An example of a phase I drug-metabolizing enzyme is the cytochrome p450 (“CYP”) superfamily, the members of which comprise the major drug-metabolizing enzymes expressed in the liver. The CYP superfamily comprises heme proteins which catalyze the oxidation and dehydrogenation of a number of endogenous and exogenous lipophilic compounds. The CYP superfamily has immense diversity in its functions, with hundreds of isoforms in many species catalyzing many types of chemical reactions. The CYP superfamily comprises at least 30 related enzymes, which are divided into different families according to their amino acid homology. Examples of CYP families include CYP families 1, 2, 3 and 4, which comprise endoplasmic reticulum proteins responsible for the metabolism of drugs and other xenobiotics. Approximately 10–15 individual gene products within these four families metabolize thousands of structurally diverse compounds. It is estimated that collectively the enzymes in the CYP superfamily participate in the metabolism of greater than 80% of all available drugs used in humans. For example, the CYP 1A subfamily comprises CYP 1A2, which metabolizes several widely used drugs, including acetaminophen, amitriptyline, caffeine, clozapine, haloperidol, imipramine, olanzapine, ondansetron, phenacetin, propafenone, propranolol, tacrine, theophylline, verapamil. In addition, CYP enzymes play additional roles in the metabolism of some endogenous substrates including prostaglandins and steroids.
Some CYP enzymes exist in a polymorphic form, meaning that a small percentage of the population possesses mutant genes that alter the activity of the enzyme, usually by diminishing or abolishing activity. For example, a genetic polymorphism has been well characterized with the CYP 2C19 and CYP 2D6 genes. Substrates of CYP 2C19 include clomipramine, diazepam, imipramine, mephenytoin, moclobemide, omeprazole, phenytoin, propranolol, and tolbutamide. Substrates of CYP 2D6 include alprenolol, amitriptyline, chlorpheniramine, clomipramine, codeine, desipramine, dextromethorphan, encainide, fluoxetine, haloperidol, imipramine, indoramin, metoprolol, nortriptyline, ondansetron, oxycodone, paroxetine, propranolol, and propafenone. Polymorphic variants of these genes metabolize these substrates at different rates, which can effect a patient's effective therapeutic dosage.
While the substrate specificity of CYPs must be very broad to accommodate the metabolism of all of these compounds, each individual CYP gene product has a narrower substrate specificity defined by its binding and catalytic sites. Drug metabolism can thereby be regulated by changes in the amount or activity of specific CYP gene products. Methods of CYP regulation include genetic differences in the expression of CYP gene products (i.e., genetic polymorphisms), inhibition of CYP metabolism by other xenobiotics that also bind to the CYP, and induction of certain CYPs by the drug itself or other xenobiotics. Inhibition and induction of CYPs is one of the most common mechanisms of adverse drug interactions. For example, the CYP3A subfamily is involved in clinically significant drug interactions involving nonsedating antihistamines and cisapride that may result in cardiac dysrhythmias. In another example, CYP3A4 and CYP1A2 enzymes are involved in drug interactions involving theophylline. In yet another example, CYP2D6 is responsible for the metabolism of many psychotherapeutic agents. Additionallly, CYP enzymes metabolize the protease inhibitors used to treat patients infected with the human immunodeficiency virus. By understanding the unique functions and characteristics of these enzymes, physicians may better anticipate and manage drug interactions and may predict or explain an individual's response to a particular therapeutic regimen.
Examples of reactions catalyzed by the CYP superfamily include peroxidative reactions utilizing peroxides as oxygen donors in hydroxylation reactions, as substrates for reductive beta-scission, and as peroxyhemiacetal intermediates in the cleavage of aldehydes to formate and alkenes. Lipid hydroperoxides undergo reductive beta-cleavage to give hydrocarbons and aldehydic acids. One of these products, trans-4-hydroxynonenal, inactivates CYP, particularly alcohol-inducible 2E1, in what may be a negative regulatory process. Although a CYP iron-oxene species is believed to be the oxygen donor in most hydroxylation reactions, an iron-peroxy species is apparently involved in the deformylation of many aldehydes with desaturation of the remaining structure, as in aromatization reactions.
Examples of drugs with oxidative metabolism associated with CYP enzymes include acetaminophen, alfentanil, alprazolam, alprenolol, amiodarone, amitriptyline, astemizole, buspirone caffeine, carbamazepine, chlorpheniramine, cisapride, clomipramine, clomipramine, clozapine, codeine, colchicine, cortisol, cyclophosphamide, cyclosporine, dapsone, desipramine, dextromethorphan, diazepam, diclofenac, diltiazem, encainide, erythromycin, estradiol, felodipine, fluoxetine, fluvastatin, haloperidol, ibuprofen, imipramine, indinavir, indomethacin, indoramin, irbesartan, lidocaine, losartan, macrolide antibiotics, mephenytoin, methadone, metoprolol, mexilitene, midazolam, moclobemide, naproxen, nefazodone, nicardipine, nifedipine, nitrendipine, nortriptyline, olanzapine, omeprazole, ondansetron, oxycodone, paclitaxel, paroxetine, phenacetin, phenytoin, piroxicam, progesterone, propafenone, propranolol, quinidine, ritonavir, saquinavir, sertraline, sildenafil, S-warfarin, tacrine, tamoxifen, tenoxicam, terfenadine, testosterone, theophylline, timolol, tolbutamide, triazolam, verapamil, and vinblastine.
Abnormal activity of phase I enzymes has been implicated in a range of human diseases. For example, enhanced CYP2D6 activity has been related to malignancies of the bladder, liver, pharynx, stomach and lungs, whereas decreased CYP2D activity has been linked to an increased risk of Parkinson's disease. Other syndromes and developmental defects associated with deficiencies in the CYP superfamily include cerebrotendinous xanthomatosis, adrenal hyperplasia, gynecomastia, and myetonic dystrophy.
The CYP superfamily a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of the CYP superfamily.
UDP-Glucuronosyltransferases
Potential drug interactions involving phase II metabolism are increasingly being recognized. An important group of phase II enzymes involved in drug metabolism are the glucuronosyltransferases, especially the UDP-glucuronyltransferase (“UGT”) superfamily. Members of the UGT superfamily catalyze the enzymatic addition of UDP glucuronic acid as a sugar donor to fat-soluble chemicals, a process which increases their solubility in water and increases their rate of excretion. In mammals, glucuronic acid is the main sugar that is used to prevent the accumulation of waste products of metabolism and fat-soluble chemicals from the environment to toxic levels in the body. Both inducers and inhibitors of glucuronosyltransferases are known and have the potential to affect the plasma concentration and actions of important drugs, including psychotropic drugs.
The UGT superfamily comprises several families of enzymes in several species defined with a nomenclature similar to that used to define members of the CYP superfamily. In animals, yeast, plants and bacteria there are at least 110 distinct known members of the UGT superfamily. As many as 33 families have been defined, with three families identified in humans. Different UGT families are defined as having <45% amino acid sequence homology; within subfamilies there is approximately 60% homology. The members of the UGT superfamily are part of a further superfamily of UDP glycosyltransferases found in animals, plants and bacteria.
The role of phase II enzymes, and of UGT enzymes in particular, is being increasingly recognized as important in psychopharmacology. UGT enzymes conjugate many important psychotropic drugs and are an important source of variability in drug response and drug interactions. For example, the benzodiazepines lorazepam, oxazepam, and temazepam undergo phase II reactions exclusively before being excreted into the urine.
Phase II enzymes metabolize and detoxify hazardous substances, such as carcinogens. The expression of genes encoding phase II enzymes is known to be up-regulated by hundreds of agents. For example, oltipraz is known to up-regulate phase II enzyme expression. Studies have demonstrated protection from the cancer-causing effects of carcinogens when selected phase II enzyme inducers are administered prior to the carcinogens. The potential use of phase II enzyme inducers in humans for prevention of cancers related to exposure to carcinogens has prompted studies aimed at understanding their molecular effects. Current biochemical and molecular biological research methodologies can be used to identify and characterize selective phase II enzyme inducers and their targets. Identification of genes responding to cancer chemopreventive agents will facilitate studies of their basic mechanism and provide insights about the relationship between gene regulation, enzyme polymorphism, and carcinogen detoxification.
Examples of drugs with conjugative metabolism associated with UGT enzymes include amitriptyline, buprenorphine, chlorpromazine, clozapine, codeine, cyproheptadine, dihydrocodeine, doxepin, imipramine, lamotrigine, lorazepam, morphine, nalorphine, naltrexone, temazepam, and valproate.
Abnormal activity of phase II enzymes has been implicated in a range of human diseases. For example, Gilbert syndrome is an autosomal dominant disorder caused by mutation in the UGT1 gene, and mutations in the UGT1A1 enzyme have been demonstrated to be responsible for Crigler-Najjar syndrome.
The UGT superfamily a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of the UGT superfamily.
For a further review of UDP-glucuronosyltransferases, see Jin et al., Biochem Biophys Res Commun 1993 Jul. 15; 194(1):496–503; Beaulieu et al., Biochem Biophys Res Commun 1998 Jul. 9; 248(1):44–50; Belanger et al., DNA Cell Biol 1997 Oct.; 16(10):1195–205; Jackson et al., Biochem J 1987 Mar. 1; 242(2):581–8; Taura et al., Biochem Biophys Res Commun 2000 Jul. 14; 273(3):1048–1052; Burchell et al., DNA Cell Biol. 10: 487–494, 1991; Krasnewich et al., Somat. Cell Molec. Genet. 13: 179–182, 1987; Monaghan et al., Genomics 23: 496–499, 1994; Monaghan et al., Genomics 13: 908–909, 1992; and Riedy et al., Pharmacogenetics 10: 251–260, 2000.
Drug-metabolizing enzymes, particularly members of the UDP-glucuronosyltransferase drug-metabolizing enzyme subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of drug-metabolizing proteins. The present invention advances the state of the art by providing a previously unidentified human drug-metabolizing proteins that have homology to members of the UDP-glucuronosyltransferase drug-metabolizing enzyme subfamily.